1. Technical Field
The present invention relates to an image reading apparatus for reading a document optically, and a focus adjusting method thereof.
2. Background Art
Concerning a focus adjusting method used for an image reading apparatus for reading a document optically, there is provided a conventional method in which a distance from a condenser lens to an optical sensor is adjusted while both the condenser lens and the optical sensor such as CCD line sensor are independently moved, and the degree of contrast of one line of an image, which has been read by the optical sensor, is measured with respect to a plurality of distances, and it is judged that the image is in focus at a position of the optical sensor at which the maximum degree of contrast is obtained. Concerning the degree of contrast, a difference between the maximum and the minimum of luminance of the image to be read, or a total of absolute values of differences between values which have been read by the adjacent photoelectric transfer elements is calculated, and it is judged that the higher the value is, the more intense the degree of contrast becomes.
However, when the above conventional method is adopted, the following disadvantages may be encountered. In order to move both the condenser lens and the optical sensor independently, the moving mechanism necessarily becomes complicated. Further, when a large black solid region or a large white solid region exists on the document, that is, when a space frequency of the document is low, there is provided a small difference between the degree of contrast when the image is in focus and the degree of contrast when the image is out of focus, by the method in which the focusing position is found by calculating the degree of contrast as described above. Accordingly, it is impossible to accurately adjust the focus.
In the above focus adjusting method, when the condenser lens is moved in order to change a distance from the document to the condenser lens, the condenser lens may be minutely shifted in a direction which is not parallel to the optical path. In this case, a reading position of the line sensor in the direction of vertical scanning is shifted from a right position. When the condenser lens is moved more accurately, the reading position is not shifted from the right position, however, the manufacturing cost is remarkably increased. Therefore, improvements in the accuracy are limited. When the reading position is shifted from the right position, the degree of contrast is compared at different positions on the document. Therefore, it is impossible to find the focal distance accurately. When the apparatus is composed in such a manner that both the condenser lens and the line sensor can be independently moved, the moving mechanism necessarily becomes complicated.
When a length of the optical path from the document to the condenser lens is changed, a discrepancy may be caused between a position on the document which is read by the same element of the line sensor before the change and a position on the document which is read by the same element of the line sensor after the change. Therefore, a range of the image to be read before the change in the optical path length is different from a range of the image to be read after the change in the optical path length. When the degree of contrast is simply found by an output signal of a pixel in the same range of the line sensor in the case of focusing, it results in a comparison of the contrast with respect to different ranges of the image in the horizontal scanning direction on the document. Therefore, it is impossible to conduct focusing accurately.
The present invention has been accomplished in view of the above circumstances. It is an object of the present invention to provide an image reading apparatus in which focusing can be accurately conducted by a simple moving mechanism. Also, it is an object of the present invention to provide a focus adjusting method thereof. It is another object of the present invention to provide an image reading apparatus in which an error caused by the deviation of a reading position in the vertical scanning direction in the case of adjusting the focus can be reduced. Also, it is another object of the present invention to provide a focus adjusting method thereof. It is still another object of the present invention to provide an image reading apparatus in which a document can be read in the same range in the horizontal scanning direction at all times so that the focus can be more accurately adjusted. Also, it is still another object of the present invention to provide a focus adjusting method thereof.
In order to accomplish the above objects, an image reading apparatus of the present invention comprises: a condenser lens for condensing light sent from a document; an optical sensor for converting light sent from the condenser lens into an electric signal; an optical path length adjusting means for integrally moving the condenser lens and the optical sensor so as to adjust an optical path length between the document and the condenser lens; a calculating means for storing a signal obtained when the optical sensor reads a reading line on the document by a plurality of optical path lengths and also for calculating a value showing the degree of focusing at each optical path length; and a control means for giving a command to the optical path length adjusting means so that the value showing the degree of focusing can be maximized. Since the condenser lens and the optical sensor are integrally moved, it is possible to conduct focusing accurately without using a complicated moving mechanism.
A focus adjusting method of an image reading apparatus the present invention comprises the steps of: (a) moving the condenser lens and the optical sensor integrally so as to adjust an optical path length between the document and the condenser lens; (b) reading luminance of a reading line on the document by the optical sensor and converting the luminance into an electric signal; (c) calculating a value showing the degree of focusing by the electric signal obtained in step (b); (d) repeating steps (a), (b) and (c) by a plurality of optical path lengths; (e) finding an optical path length by which the value showing the degree of focusing is maximized; (f) adjusting an optical path length so that it can be the optical path length found in step (e).
An image reading apparatus and its focus adjusting method of the present invention use a line sensor having a plurality of photoelectric transfer elements as an optical sensor.
In an image reading apparatus and its focus adjusting method of the present invention, a value showing the degree of focusing is a total of the square root of the sum of the square of a predetermined value corresponding to a distance between the adjacent reading positions to be read by the optical sensor, and the square of a difference of luminance signals at the adjacent reading positions. Accordingly, even in the case of an image having a large solid black region and large solid white region, that is, even in the case of an image, the space frequency of which is low, it is possible to conduct focusing accurately. A square root of the sum of the square of a distance between the reading positions adjacent to each reading position and the square of a difference of luminance signals, which have been read, corresponds to a length of a straight line which connects points representing an intensity of luminance signal at the reading position on a graph, the horizontal axis of which is a reading position and the vertical axis of which is an intensity of the luminance signal which has been read, and a sum of the square roots corresponds to a total length of the lines on the broken line graph.
An image reading apparatus of the present invention comprises: a condenser lens for condensing light sent from a document; a line sensor having a plurality of photoelectric transfer elements for converting light sent from the condenser lens into an electric signal; an optical path length adjusting means for adjusting an optical path length between the document and the condenser lens; a calculating means for storing a signal obtained when the line sensor reads a plurality of reading lines on the document and also for calculating a value showing the degree of focusing at each line; and a control means for giving a command to the optical path length adjusting means so that a sum of the values showing the degree of focusing on a plurality of lines can be maximized.
Accordingly, even when the document reading position deviates in the vertical scanning direction in the process of adjusting the focus, an error caused by the deviation can be reduced by using a total of the values showing the degree of focusing on a plurality of lines. Therefore, focusing can be accurately conducted.
A focus adjusting method of an image reading apparatus of the present invention comprises the steps of: (a) adjusting an optical path length between the condenser lens and the document; (b) reading a plurality of lines on the document by the line sensor and calculating a value showing the degree of focusing on each line; (c) repeating steps (a) and (b) by a plurality of optical path lengths; (d) finding an optical path length by which the value showing the degree of focusing is maximized on the plurality of lines; and (e) adjusting an optical path length between the condenser lens and the document so that it can be the optical path length found in step (d).
In an image reading apparatus and its focus adjusting method of the present invention, the optical path length adjusting means is a means in which the condenser lens and the line sensor are integrally moved, and the optical path length adjusting step is a step in which the condenser lens and the line sensor are integrally moved.
In the focus adjusting method of the present invention, a plurality of lines are read at intervals in step (b). Since a plurality of lines are read at intervals in step (b), it is possible to reduce a period of time required for reading while a wide reading range is ensured in the step of adjusting the focus. Therefore, it is possible to adjust the focus in a short period of time. Concerning the value showing the degree of focusing, when a broken line graph is considered, the horizontal axis of which represents a reading position of the document in the horizontal scanning direction and the vertical axis of which represents an output value, a total length of the broken lines or a value of the degree of contrast of the signal which has been obtained by reading the document may be used as a value showing the degree of focusing.
An image reading apparatus of the present invention comprises: a condenser lens for condensing light sent from a document; a line sensor having a plurality of photoelectric transfer elements for converting light sent from the condenser lens into an electric signal; an optical path length adjusting means for adjusting an optical path length between the document and the condenser lens; and a means for correcting positions of the photoelectric transfer elements on the line sensor used for reading according to the optical path length. Due to the foregoing, it is possible to read an image of the same range of the line sensor in the horizontal scanning direction before and after a change in the optical path length. Concerning the value showing the degree of focusing, when a broken line graph is considered, the horizontal axis of which represents a reading position of the document in the horizontal scanning direction and the vertical axis of which represents an output value, a total length of the broken lines or a value of the degree of contrast of the signal which has been obtained by reading the document may be used as the value showing the degree of focusing.
A focus adjusting method of an image reading apparatus of the present invention comprises the steps of: (a) adjusting an optical path length between the condenser lens and the document; (b) correcting positions of the photoelectric transfer elements on the line sensor used for reading according to the optical path length; (c) calculating a value showing the degree of focusing when the document is read by the photoelectric transfer elements located at the corrected positions on the line sensor; (d) repeating steps (a), (b) and (c) by a plurality of optical path lengths; (e) finding an optical path length by which the value showing the degree of focusing is maximized; and (f) adjusting an optical path length between the condenser lens and the document so that it can be the optical path length found in step (e).
An image reading apparatus of the present invention further comprises: a means for calculating a value showing the degree of focusing on a plurality of optical path lengths when the document is read by the photoelectric transfer elements, the positions of which have been corrected, on the line sensor; and a control means for giving a command to the optical path length adjusting means so that the value showing the degree of focusing can be maximized. Due to the foregoing, even when an optical path length between the document and the condenser lens is changed in the process of adjusting the focus, it is possible to read the same range on the document in the horizontal scanning direction. Therefore, focusing can be more accurately conducted.